1. Field of the Invention
The invention relates to an electronic arrangement for generating an amplitude and phase-modulated carrier signal.
In radio transmission a great diversity of amplitude and phase-modulated carrier signals are employed, such as single-sideband signals, so-called "Offset Quadrature Phase Shift Keying" signals, abbreviated to OQPSK-signals, and so on.
The most important advantage of signals modulated in this manner, compared to signals of which only the amplitude, the phase or the frequency have been modulated resides in the fact that the bandwidth required for the transmission of the same quantity of information in amplitude and phase-modulated signals is less than for signals of which only the amplitude, the phase or the frequency is modulated. So, a carrier signal whose amplitude and phase are modulated by an information signal, for example a single-sideband signal and a 4-phase modulated signal with limited bandwidth respectively, or a OQPSK-modulated signal requires half the bandwidth of a carrier signal whose amplitude is modulated by the same information signal, such as a double-sideband signal and a narrower bandwidth, respectively than a carrier signal without band limitation which is 4-phase modulated by the same information signal, such as what are commonly referred to as a "Fast Frequency Shift Keying"-signal (FFSK) or a "Minimum Shift Keying"-signal (MSK).
2. Description of the Prior Art
Existing systems often employ only phase or frequency modulation, as the case may be, as the distortion of non-linear, efficient power amplifiers is then only located around multiples of the carrier frequencies. This distortion can be eliminated in a simple manner subsequently by means of filtering. If, however, amplitude modulation is also present then distortion is produced in the information band itself. A low distortion level is then realized by means of extremely linear amplifiers. These amplifiers are expensive and have a low efficiency.
It is an object of the invention to provide an electronic circuit arrangement for an amplitude and phase-modulated signal which obviates the drawbacks mentioned in the foregoing to a very high extent while maintaining the narrow-band character and which in addition makes it possible to realize the modulation at a low-signal level by means of integrated circuits and to form therefrom a high-power output signal in a very simple and efficient manner.